In Rel-10 LTE, the UE can be configured with multiple downlink carrier frequencies for downlink carrier aggregation and only one uplink carrier frequency. The primary component carrier comprises of a pair of downlink and uplink carriers (on different frequencies for FDD system and on the same frequency for TDD system), while the secondary component carrier may comprise only a single downlink carrier frequency with no uplink carrier frequency. Layer 1 uplink control information associated with the secondary component carrier is always transmitted on the primary component carrier.
There is benefit to enable aggregation of two or more downlink carrier frequencies for a UE where different carrier frequency is associated with different eNodeB and the eNodeBs concerned may not be co-located at the same site (inter-site inter-eNodeB carrier aggregation). This is also known as non-co-channel dual connectivity. A certain deployment scenario may have different neighboring eNodeB configured with different downlink and uplink carrier pair. A certain deployment scenario may also have the eNodeBs interconnected with slow backhaul (e.g. 40 ms on-way transmission latency).
When inter-eNodeB carrier aggregation is configured, the traffic to the UE may predominantly flow through a particular carrier. In one example, traffic with best effort QoS may predominantly flow through one carrier while traffic with stricter QoS may predominantly flow through another carrier. In another example, traffic may predominantly flow through one carrier because the corresponding path loss may be lower.
When the UE is configured with carrier aggregation where a first eNodeB is associated with the primary component carrier and a second eNodeB is associated with the secondary component carrier, it is not desirable for the UE to transmit Layer 1 uplink control information corresponding to the second component carrier to the primary component carrier due to the excessive latency incurred for the first eNodeB to transmit the Layer 1 uplink control information corresponding to the second component carrier to the second eNodeB over the X2 interface. Therefore, there is a need to transmit uplink control information and uplink data associated with an eNodeB directly over the air to the eNodeB concerned when inter-site inter-eNodeB carrier aggregation is configured.